Marijuana is the most commonly used illicit drug in the US. In the past few years there has also been increased interest in using marijuana for symptom relief in specific chronic illnesses. Furthermore, it has been proposed that cannabis use predisposes individuals to subsequent opiate abuse. Two questions that are important for considering the implications of these observations are: What is the mechanism underlying the development of tolerance to the psychoactive and therapeutic effects of cannabis? What is the nature of the interaction between cannabis and opiates in the brain? In the previous decade substantial progress has been made in understanding how cannabinoids, the primary active components of marijuana, produce their psychoactive effects. This puts us in a great position to explore the consequences of chronic cannabinoid use and the interactions of cannabinoids with other neuromodulators. Despite progress in understanding the acute effects of cannabinoids in the brain, our understanding of the chronic effects of cannabinoids and the interaction of the endogenous cannabinoid system with opiates remains primitive. While it is clear that tolerance to cannabinoids does develop, and a distinct withdrawal syndrome in tolerant animals can be precipitated, the cellular and molecular mechanisms underlying these processes remain unknown. In addition, a recent report using a constitutive deletion of the CB1 gene suggests that CB1 receptors may be required for opiate reinforcement. This finding has important implications for the development of opiate addiction. Using a genetic approach we will address the molecular mechanisms underlying the development of tolerance to cannabinoids and the impact of the endogenous cannabinoid system on the rewarding properties of opiates. The two specific aims of this proposal are:1. Is tolerance to cannabinoids due to CB1 receptor phosphorylation? 2. What role do CB1 receptors play in opiate reinforcement? Accomplishing these specific aims will substantially advance our understanding of the mechanisms underlying the development of tolerance to cannabinoids and the role of endogenous cannabinoids in the development of opiate dependence. The results of these studies will also give insight into the importance of receptor phosphorylation for the development of tolerance in other G protein-coupled receptor-mediated responses. Furthermore, the transgenic mice developed in these studies will be a valuable resource for other NIDA-funded researchers in the cannabinoid field.